From the start of the coronavirus outbreak, experts warned the illness was spread when an infected person expelled respiratory droplets while coughing or sneezing.
But in April, the Centers for Disease Control and Prevention — which emphasizes it’s still learning about how COVID-19 is spread — added talking as one of the ways an infected person can transmit the disease.
Now a new study explains why that might be.
"Speech droplets" produced by people who could be infected but don't show any symptoms can linger in the air for eight to 14 minutes, researchers reported on May 13 in The Proceedings of the National Academy of Sciences.
Scientists at the National Institute of Diabetes and Digestive and Kidney Diseases and the University of Pennsylvania asked people to repeat the words "stay healthy" for 25 seconds, choosing that particular phrase because the "th" in the word "healthy" was found to be "an efficient generator of oral fluid speech droplets."
The volunteers said the words into a box and the results were observed using a laser light scattering technique, which detected 2,600 small droplets per second of speaking, with loud speech producing even more.
Speech generates airborne droplets that can remain suspended for minutes and are "eminently capable of transmitting disease in confined spaces," the authors wrote.
"There is a substantial probability that normal speaking causes airborne virus transmission in confined environments."
Research provided to the White House in April in a letter suggested the same.
“You may generate droplets that are invisible — they're so tiny that you can’t see them, but they're certainly big enough to carry a virus, if you happen to have it in you, when you're talking,” said Dr. Harvey Fineberg, president of the Gordon and Betty Moore Foundation in Palo Alto, California.
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Joseph Fair, a virologist and NBC News contributor who is now himself sick with COVID-19, said that research was based on the most critically ill COVID-19 patients in hospitals who had labored breathing. Researchers detected evidence of virus, but that doesn't mean they found infectious virus, he added.
The White House declined to comment on the letter, but a senior administration official noted Dr. Anthony Fauci has made similar comments in the past.
“There is some degree of asymptotic transmissibility. It's still not quite clear exactly what that is,” Fauci said during a coronavirus task force briefing on March 20.
The developments led the CDC to recommend that people should wear face coverings in public, which helps prevent asymptomatic carriers of coronavirus from transmitting it to others, based on data from studies in China, Iceland and the U.S., Fair said.
Handkerchiefs or "anything you can tie around your face" can serve as a sufficient mask, he added. They should particularly be worn in places like grocery stores and pharmacies where people need to go and will be around others.
Meanwhile, doctors and nurses across the country are working tirelessly to treat patients and figure out the disease. Unusual symptoms — like the loss of taste or smell, pink eye and digestive issues — are also emerging.
Fauci told TODAY’s Savannah Guthrie that even he has been puzzled.
“It is very strange how one individual can get infected and have either mild or no symptoms, and another individual could rapidly deteriorate with viral pneumonia and respiratory failure,” he said on Thursday.
“I've been doing infectious diseases now for almost 50 years, and I can tell you, I don't fully understand exactly what the mechanism that is, and we really need to figure it out.”
This is an updated version of a story originally published on April 3, 2020.